Synthesis gas containing hydrogen and carbon oxides is an important feedstock for the production of a wide range of chemical products. Synthesis gas mixtures with the proper ratios of hydrogen to carbon monoxide are reacted catalytically to produce liquid hydrocarbons and oxygenated organic compounds including methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates. Liquids produced by synthesis gas conversion are valuable as fuels, chemical intermediates, and final chemical products. High purity hydrogen and carbon monoxide can be recovered by further processing and separation of synthesis gas. The cost of generating the synthesis gas usually is the largest part of the total cost of these products.
Two major reaction routes are used for synthesis gas production--steam reforming of light hydrocarbons, primarily natural gas, naphtha, and refinery offgases, and the partial oxidation of carbon-containing feedstocks ranging from natural gas to high molecular weight liquid or solid carbonaceous materials. Autothermal reforming is an alternative process using light hydrocarbon feed in which both partial oxidation and steam reforming reactions occur in a single reactor. In the various versions of autothermal reforming, feed gas is partially oxidized in a specially-designed burner and the resulting hot gas passes through a catalyst bed where steam reforming and CO.sub.2 reforming occur. Newer synthesis gas generation processes include various heat exchange reformers such as gas heated reforming (GHR) developed by ICI, the SMART reformer by KTI, and the CAR reformer by UHDE; the improved Texaco gasification process (TGP) included in their HyTEX.TM. hydrogen production system; Haldor-Topsoe's HERMES process; the Shell gasification process (SGP); Exxon's fluidized bed synthesis gas process; and Kellogg's KRES process.
The state of the art in commercial synthesis gas generation technology is summarized in representative survey articles including "Steam Reforming--Opportunities and Limits of the Technology" by J. Rostrup-Nielsen et al, presented at the NATO ASI Study on Chemical Reactor Technology for Environmentally Safe Reactors and Predictors, Aug. 25-Sep. 5, 1991, Ontario, Canada; "Improve Synthesis gas Production Using Autothermal Reforming" by T. S. Christiansen et al, Hydrocarbon Processing, March 1994, pp. 39-46; "Evaluation of Natural Gas Based Synthesis Gas Production Technologies" by T. Sundset et al, Catalysis Today, 21 (1994), pp. 269-278; "Production of Synthesis Gas by Partial Oxidation of Hydrocarbons" by C. L. Reed et al, presented at the 86.sup.th National AlChE meeting, Houston, Texas, Apr. 1-5, 1979; "Texaco's HyTEX.TM. Process for High Pressure Hydrogen Production" by F. Fong, presented at the KTI Symposium, Apr. 27, 1993, Caracas, Venezuela; and "Custom-Made Synthesis Gas Using Texaco's Partial Oxidation Technology" by P. J. Osterrieth et al, presented at the AlChE Spring National Meeting, New Orleans, LA, Mar. 9, 1988.
Staged steam-methane reforming processes are used to upgrade the performance of existing plants and for the design of more efficient new plants for producing synthesis gas. One type of staged reforming utilizes a prereformer, typically an adiabatic reforming reactor containing a highly active nickel catalyst, to reform heavier hydrocarbons in the feedstock (and a portion of the methane, if present) to yield a mixture of methane, hydrogen, carbon monoxide, carbon dioxide, and steam. This prereforming product is then further processed in a fired tubular reformer to produce a raw synthesis gas product. Another type of staged reformer process utilizes a gas heated reformer (GHR) followed by an autothermal reformer. The GHR is a type of heat exchange reformer in which the hot raw synthesis gas from the autothermal reformer furnishes the heat for the first reforming stage in the GHR.
Staged reforming processes are described in papers entitled "The Application of Pre-Reforming Technology in the Production of Hydrogen" by B. J. Cromarty et al, presented at the NPRA Annual Meeting, Mar. 21-23, 1993, San Antonio, Tex.; "The Benefits of Pre-reforming in Hydrogen Production Plants" by J. M. Foreman et al, presented at the World Hydrogen Conference, June 1992; and "Modern Aspects of Steam Reforming for Hydrogen Plants" by B. J. Cromarty, presented at the World Hydrogen Conference, June 1992. Gas heated reforming is described in a paper by K. J. Elkins et al entitled "The ICI Gas-Heated Reformer (GHR) System" presented at the Nitrogen '91 International Conference, Copenhagen, June 1992.
Other combinations of steam reforming and autothermal reforming are used in synthesis gas production. In the production of ammonia synthesis gas, for example, a combination of steps called primary reforming and secondary reforming is used in which natural gas is steam reformed and the resulting intermediate product is further converted in an air-fired autothermal reforming reactor to yield raw ammonia synthesis gas containing hydrogen, nitrogen, and carbon monoxide. Primary steam reforming followed by oxygen secondary reforming (autothermal reforming) is used in the production of synthesis gas containing hydrogen and carbon monoxide in which secondary reforming is carried out in an oxygen-fired autothermal reformer. Primary steam reforming can be carried out in a fired tubular reformer.
In the commercial processes described above which utilizes an autothermal reforming step, oxygen is required and is typically supplied at purities of 95 to 99.9 vol %. Oxygen is obtained by the separation of air using known methods, usually the low-temperature distillation of air for larger volumes and pressure swing adsorption for smaller volumes.
The conversion of synthesis gas into a wide variety of products is well known in the art as described in compendia such as the Kirk-Othmer Encyclopedia of Chemical Technology, 4.sup.th Edition, 1991, Wiley-Interscience, New York. Two of the largest volume consumers of synthesis gas in the chemical process industries are the Fischer-Tropsch process for the synthesis of higher molecular weight hydrocarbons and the various gas-phase and liquid-phase methanol synthesis processes. These high-volume products find use as fuels and as chemical intermediates for further product synthesis. The well-known Fischer-Tropsch process is described widely in the art, for example in an article entitled "Fischer-Tropsch Synthesis" by B. Bussemeier et al in the Encyclopedia of Chemical Process and Design, 22, p 81-119 (1985).
An alternative technology for synthesis gas production is in the early stages of development in which oxygen for the partial oxidation reactions is provided in situ by the separation of air at high temperatures using ceramic, ceramic-metal, or ceramic-ceramic composite membranes which conduct both electronic species and oxygen ions. These membranes are included in a broad class of membranes known generically as ion transport membranes, and form a specific class of ion transport membranes known collectively as mixed conducting membranes which conduct both electronic species and oxygen ions. These membranes can be used optionally in combination with appropriate catalysts to produce synthesis gas in a membrane reactor without the need for a separate oxygen production unit. The reactor is characterized by one or more reaction zones wherein each zone comprises a mixed conducting membrane which separates the zone into an oxidant side and a reactant side.
An oxygen-containing gas mixture, typically air, is contacted with the oxidant side of the membrane and oxygen gas reacts with electronic species to form oxygen ions which permeate through the membrane material. A reactant gas containing methane and other low molecular weight hydrocarbons flows across the reactant side of the membrane. Oxygen (as defined later) on the reactant side of the membrane reacts with components in the reactant gas to form synthesis gas containing hydrogen and carbon monoxide. A catalyst to promote the transfer of oxygen into the membrane can be applied to the surface of the membrane on the oxidant side. A catalyst to promote the conversion of reactant gas components to synthesis gas may be applied to the surface of the reactant side of the membrane; alternatively or additionally, a granular form of the catalyst may be placed adjacent to the membrane surface. Catalysts which promote the conversion of hydrocarbons, steam, and carbon dioxide to synthesis gas are well-known in the art.
Numerous reactors and compositions of mixed conducting membranes suitable for this purpose have been disclosed in the art. Membrane reactors and methods of operating such reactors for the selective oxidation of hydrocarbons are disclosed in related U.S. Pat. Nos. 5,306,411 and 5,591,315. Ceramic membranes with wide ranges of compositions are described which promote the transfer of oxygen from an oxygen-containing gas and reaction of the transferred oxygen with a methane-containing gas to form synthesis gas. Mixed conductors having a perovskite structure are utilized for the membrane material; alternatively multiphase solids are used as dual conductors wherein one phase conducts oxygen ions and another conducts electronic species. A membrane reactor to produce synthesis gas is disclosed which operates at a temperature in the range of 1000 to 1400.degree. C., wherein the reactor may be heated to the desired temperature and the temperature maintained during reaction by external heating and/or exothermic heat from the chemical reactions which occur. In one general embodiment, it is disclosed that the process is conducted at temperatures within the range of 1000 to 1300.degree. C. Experimental results are reported for oxygen flux and synthesis gas production in an isothermal laboratory reactor using a dual-conducting membrane at a constant temperature of 1100.degree. C. Non-combustible diluents such as nitrogen, argon, helium, and other gases may be present in the reactor feed and do not interfere with the desired chemical reactions. Steam if present in the reactor feed is stated to be an inert gas or diluent.
In a paper entitled "Ceramic Membranes for Methane Conversion" presented at the Coal Liquefaction and Gas Conversion Contractors, Review Conference, Sep. 7-8, 1994, Pittsburgh, Pa., U. Balachandran et al describe the fabrication of long tubes of Sr--Co.sub.0.5 --Fe--O.sub.x membranes and the operation of these tubes for conversion of methane to synthesis gas in laboratory reactors at 850.degree. C.
U.S. Pat. No. 4,793,904 discloses the use of a solid electrolyte membrane with conductive coatings on both sides which are optionally connected by an external circuit. The membrane is used in an electrolytic cell at temperatures in the range of 1050 to 1300.degree. C. to convert methane to synthesis gas at a pressure of about 0.1 to about 100 atmospheres. Experimental results are presented for the conversion of methane to synthesis gas components in a reactor cell with an yttria-stabilized zirconia membrane having platinum electrodes optionally using an external electrical circuit. The reactor cell was operated isothermally at a temperature of 800, 1000, or 1100.degree. C.
Related U.S. Pat. Nos. 5,356,728 and 5,580,497 disclose cross-flow electrochemical reactor cells and the operation of these cells to produce synthesis gas from methane and other light hydrocarbons. Mixed conducting membranes made of mixed oxide materials are disclosed for use in the crossflow reactor cells. The production of synthesis gas by the partial oxidation of hydrocarbons is disclosed using reactor temperatures of about 1000 to 1400.degree. C. or alternatively in the range of about 450 to 1250.degree. C. Experimental results are reported for synthesis gas production in isothermal tubular laboratory reactors at constant temperatures in the range of 450 to 850.degree. C. A pressure in the ceramic tube reactor, typically about 6 inches of water head, was maintained by means of a downstream water bubbler.
U.S. Pat. No. 5,276,237 discloses the partial oxidation of methane to synthesis gas using a mixed metal oxide membrane comprising alumina with multivalent activator metals such as yttrium and barium. A process concept is disclosed with low oxygen recovery to facilitate heat removal and maintain a high oxygen partial pressure driving force. The partial oxidation reactions were carried out at a temperature in the range of about 500 to about 1200.degree. C., and the temperature on the oxygen side of the membrane is described to be at most only a few degrees less than the reaction temperature on the reactant side of the membrane.
The practical application of mixed conducting membranes to produce synthesis gas will require reactor modules having a plurality of individual membranes with appropriate inlet and outlet flow manifolds to transport feed and product gas streams. Such modules provide the large membrane surface area required to produce commercial volumes of synthesis gas product. Module designs have been disclosed in the art which address this requirement. Previously-cited U.S. Pat. Nos. 5,356,728 and 5,580,497 describe one type of crossflow membrane reactor which has hollow ceramic blades positioned across a gas stream flow or a stack of crossed hollow ceramic blades containing channels for gas flow. Alternatively, the crossflow reactor can be fabricated in the form of a monolithic core with appropriate inlet and outlet manifolding. U.S. Pat. No. 4,791,079 discloses membrane module designs for mixed conducting membrane reactors for the oxidative coupling of methane to produce higher hydrocarbons, hydrogen, and carbon oxides.
A planar membrane module is described in U.S. Pat. No. 5,681,373 which contains a plurality of planar units each of which comprises a channel-free porous support with an outer layer of mixed conducting oxide material. An oxygen-containing gas is passed through the porous supports and permeated oxygen reacts with light hydrocarbons at the outer layer of the mixed conducting oxide material. The module is heated to a temperature ranging from about 300 to 1200.degree. C. for continuous production of synthesis gas. U.S. Pat. No. 5,599,383 discloses a tubular solid state membrane module having a plurality of mixed conducting tubes each of which contains inner porous material which supports the tube walls and allows gas flow within the tube. The module can be used to produce synthesis gas wherein an oxygen-containing gas is passed through the inside of the tubes and a hydrocarbon-containing gas is passed over the outside of the tubes. The module is heated to a temperature ranging from 300 to 1200.degree. C., the oxygen-containing gas is passed through the tubes, and the hydrocarbon-containing gas is passed over the outside of the tubes. Oxygen permeates through the mixed conducting tube walls and reacts with the hydrocarbon under controlled conditions to produce synthesis gas containing hydrogen and carbon monoxide. A catalyst to promote the formation of synthesis gas may be applied to the outer surface of the tubes.
The background art summarized above characterizes the temperatures and pressures in mixed conducting membrane reactors for synthesis gas production in general non-spatial terms, that is, differences in temperature and pressure as a function of reactor geometry are not considered. All of the above disclosures teach the operation of reactors at a single temperature, i.e., as isothermal reactors, particularly for laboratory-scale reactors. In some cases, general temperature ranges are disclosed for reactor operation, but no information is offered regarding how the temperature varies with reactor geometry. In all cases, gas pressures are reported as single pressures independent of geometry, and no pressure differences between the oxidant (air) side and the hydrocarbon (fuel) side are disclosed.
C.-Y. Tsai et al describe a nonisothermal, two-dimensional computational model of a mixed conducting membrane reactor using a perovskite membrane for the partial oxidation of methane to synthesis gas. This work is presented in related publications entitled "Simulation of a Nonisothermal Catalytic Membrane Reactor for Methane Partial Oxidation to Synthesis gas" in the Proceedings of the Third International Conference on Inorganic Membranes, Worcester Mass., Jul. 10-14, 1994, and "Modeling and Simulation of a Nonisothermal Catalytic Membrane Reactor" in Chem. Eng Comm., 1995, Vol. 134, pp. 107-132. The simulation describes the effects of gas flow rate, reactor length, and membrane thickness on methane conversion and synthesis gas selectivity for a tubular reactor configuration with air on the shell side. Temperature profiles as a function of axial reactor position are also presented. Key parameters are held constant for all simulation cases; in particular, the pressure for both shell and tube sides of the reactor is specified at 1 atm and the inlet temperature is specified at 800.degree. C. Additional discussion of experimental and computational work on topics in these two publications is presented in the doctoral thesis by C.-Y. Tsai entitled "Perovskite Dense Membrane Reactors for the Partial Oxidation of Methane to Synthesis Gas", May 1996, Worcester Polytechnic Institute (available through UMI Dissertation Services).
Synthesis gas produced by mixed conducting membrane reactors will find uses similar to synthesis gas produced by the conventional processes earlier described. However, methods of integrating mixed conducting membrane reactors with downstream synthesis gas consuming processes may involve different criteria than the integration of conventional synthesis gas generation processes with synthesis gas consumers.
The successful design and operation of synthesis gas production systems which utilize mixed conducting membrane reactors will depend upon the proper integration of the reactors with upstream and downstream gas processing systems. Such downstream gas processing systems include the conversion of synthesis gas into liquid hydrocarbons and oxygenated organic compounds by the processes described above. The invention described below and defined in the claims which follow addresses criteria for integrating downstream synthesis gas conversion processes, particularly for hydrocarbon fuel production, with synthesis gas production by mixed conducting membrane reactor systems.